South Korea has vowed to "punish the enemy" as hundreds of troops, fighter jets, tanks and attack helicopters prepared for massive new drills near the heavily armed border a month after a deadly North Korean artillery attack.

South Korea has vowed to "punish the enemy" as hundreds of troops, fighter jets, tanks and attack helicopters prepared for massive new drills near the heavily armed border a month after a deadly North Korean artillery attack.

Although the North backed down from its threat to retaliate over South Korean drills on Monday in west coast waters claimed by both countries, South Korean forces have been on high alert this week, warning of surprise attacks.

The North responded to an artillery drill on South Korea's front-line Yeonpyeong Island on November 23 with an artillery bombardment that killed four, including two civilians.

The North has made some conciliatory gestures in recent days - telling a visiting US governor that it might allow international nuclear inspections of its atomic programs - but Seoul appears unmoved and is bracing for possible aggression.

"We will completely punish the enemy if it provokes us again like the shelling of Yeonpyeong Island," said Brig Gen Ju Eun-sik, chief of the army's 1st armoured brigade.

South Korea's navy has begun its annual four-day firing and anti-submarine exercises off the country's less-tense east coast.

The disputed western sea border has been the site of most of the Koreas' recent military skirmishes, including last month's artillery bombardment. But the east coast was used by the North as a submarine route for communist agents to infiltrate South Korea in the past.

Meanwhile, on Tuesday, a church illuminated a huge steel Christmas tree standing atop a South Korean peak that overlooks North Korean border towns, resuming a tradition condemned in Pyongyang as propaganda.

The lighting - which needed government permission - was a sign that President Lee Myung-bak's administration is serious about countering the North's aggression with measures of its own in the wake of the North's artillery bombardment. The North warned the tree could trigger bloodshed on the peninsula.

For decades, the rival Koreas have fought an ideological war, using leaflets, loudspeakers and radio broadcasts across the border. At the height of the propaganda, South Korea's military speakers blared messages near the border 20 hours a day, officials say.